The traditional description of thalamo-cortical pathways in the mammalian brain is that cytoarchitecturally defined nuclear groups in the thalamus project to cytoarchitecturally defined cortical areas. Recent studies from several laboratories, including our own, have demonstrated that the classically derived description of thalamo-cortical organization is incorrect. In fact, the thalamic cell and terminal fields we have studied are intimately related to brainstem input and freely cross major fiber systems to include two or more thalamic nuclear groups. In addition, data from our lab suggests the existance of multiple independent thalamo-cortical projections interconnecting a single classically defined subdivision of the thalamus with a single classically defined subdivision of cortex. For this project we propose to study the anatomical organization of thalamo-cortical systems by extending our work in the visual system to include the auditory system. Specifically, we propose to precisely define the nuclear origin, distribution and topographic organization of cell and terminal fields in the posterior thalamus that project to all specialized subdivisions of the superior temporal lobe. This work has not been performed in the old world macaque using any of the modern tracing techniques (i.e., HRP, tritiated amino acids, etc.). We also propose to precisely define the origin and terminal distribution of brainstem efferents to the medial geniculate and there relation to thalamo-cortical organization. We will relate these findings in the auditory system with our recent work in the visual system. We are also proposing to continue our work on the organization of thalamo-cortical organization of the visual system. We will verify (falsify) the existance of mlultiple independent thalamo-cortical connections. We believe that these independent thalamo-cortical projections terminate in separate cortical layers. We will verify this hypothesis. If we demonstrate that independent thalamo-cortical systems exist, we will define their organization and distribution. It is expected that this work will greatly enhance our understanding of the structural and functional organization of these pathways and will require modification of traditional theories regarding the organization of nerve pathways interconnecting the posterior thalamus and cortex.